Wild herb covers area at head of valley (6)
I believe the answer is:
savage
'wild' is the definition.
(I know that wild can be written as savage)
'herb covers area at head of valley' is the wordplay.
'herb' becomes 'sage' (sage is a kind of herb).
'covers' means one lot of letters goes inside another.
'area' becomes 'a' (maths abbreviation).
'at' means one lot of letters go next to another (I've seen this in other clues).
'head of' suggests taking the first letters.
The initial letter of 'valley' is 'v'.
'a' put after 'v' is 'va'.
'sage' placed around 'va' is 'SAVAGE'.
(Other definitions for savage that I've seen before include "Fierce, uncontrolled" , "Brutal" , "Viciously criticise" , "Uncivilised, vicious" , "Barbaric, uncivilised" .)